This invention pertains to methods for manufacturing magnetic disks comprising carbon protective overcoats and the resulting magnetic disks.
FIG. 1 illustrates in cross section a magnetic disk 10 in a disk drive 12. Magnetic disk 10 comprises a substrate 14 (e.g. glass, glass ceramic, or NiP-plated aluminum), an underlayer 16 (e.g. Cr, a Cr alloy, NiP, NiAl or other appropriate material), a magnetic layer 18 (e.g. a Co alloy), and a protective overcoat 20 (e.g. hydrogen-doped carbon, nitrogen-doped carbon, or carbon doped with both hydrogen and nitrogen). A lubricant layer 22 (e.g. perfluoropolyether) is applied to protective overcoat 20.
Magnetic disk 10 is mounted on a spindle that is rotated by a motor 24. A read-write head 26, mounted on a suspension 28, “flies” above the rotating disk. Head 26 comprises a slider including a hard Al2O3-TiC body 30 with a read-write element 32 formed on the trailing edge thereof. A carbon overcoat 34 is formed on the bottom surface (the air bearing surface) of head 26 for tribological purposes.
Magnetic layer 18 performs the function of storing data. Overcoat 20 performs several functions:    a) It prevents corrosion of magnetic layer 18.    b) It is hard, and prevents mechanical damage of magnetic layer 18.    c) It exhibits low static and dynamic friction.    d) It holds lubricant layer 22 on disk 10.    e) It prevents wear of disk 10.
Industry has devoted a large amount of time and effort trying to form appropriate carbon films to be deposited on magnetic disks as protective layers. For example, F. K. King, “Datapoint Thin Film Media”, IEEE Trans. Magn., Jul. 1982, discuses sputtering carbon on a magnetic disk. U.S. Pat. No. 5,045,165, issued to Yamashita, discusses sputtering a hydrogen-doped carbon film on a magnetic disk to prevent wear and corrosion. Yamashita teaches that the hydrogen enhances wear resistance of the carbon. European Patent Application EP 0 547 820 discusses sputtering a nitrogen-doped carbon film on a magnetic disk. The '820 application states that the nitrogen reduces stress in the carbon, and reduces the likelihood that the carbon will delaminate from the disk. U.S. Pat. No. 5,837,357 discusses a magnetic disk comprising a hydrogen-doped carbon film covered by a nitrogen-doped carbon film. U.S. Pat. No. 5,232,570 also discusses sputtering carbon on the magnetic disk in the presence of nitrogen. Other references pertaining to carbon overcoats for magnetic disks include U.S. Pat. No. 5,855,746 and PCT Patent Application WO 99/03099. This list is by no means exhaustive.
Protective carbon overcoats for magnetic disks are typically formed by sputtering. Because of the way in which they are formed, they comprise mostly SP2 carbon. Industry has been using such carbon films for many years, and has considerable experience with these films. Thus, various types of lubricants have been developed which can be applied to predominantly SP2 carbon films to cause these films to exhibit low friction and stiction. (As used herein, the term “predominantly SP2 carbon” means that of the carbon bonds in the film, more of those bonds are SP2 than any other type of bond. Similarly, “predominantly SP3 carbon” means that of the carbon bonds in the film, more are SP3 than any other type of bond.)
Recently, Komag (the assignee of the present invention) developed a new type of carbon overcoat comprising more than 70% SP3 carbon. This type of carbon overcoat is described by Wen Hong Liu et al. in U.S. patent application Ser. No. 09/298,107, filed on Apr. 22, 1999, incorporated herein by reference. The '107 carbon is deposited by applying a novel voltage waveform to carbon sputtering targets. It has been discovered that this carbon overcoat is extremely hard and scratch resistant.
There are other types of carbon overcoats that have high SP3 contents. In particular, one can form a carbon film using chemical vapor deposition, ion beam deposition, or cathodic arc deposition. Weiler et at., “Deposition of Tetrahedral Hydrogenated Amorphous Carbon Using a Novel Electron Cyclotron Wave Resonance Reactor”, Applied Physics Letters, Vol. 72, No. 11, Mar. 16, 1998, discusses ion beam deposition of carbon. Kang, et al., “Evaluation of the Ion Bombardment Energy for Growing Diamondlike Carbon in an Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition”, J. Vac. Sci. Technol. A. 16(4), July/August 1998, discusses using chemical vapor deposition to fonn a carbon film. J. Robertson, “Ultrathin Carbon Overcoats for Magnetic Storage Technology”, TRIB-Vol. 9, Proceedings of the Symposium on Interface Technology Towards 100 Gbit/in2, ASME 1999 discusses cathodic arc deposition. Other references include U.S. Pat. No. 5,476,691; Brown, “Vacuum Arc Ion Sources”, Rev. Sci. Instrum. 65(10), October 1994, Sanders, et al., “Coating Technology Based on the Vacuum Arc—a Review”, IEEE Transactions on Plasma Science, Vol. 18, No. 6, 1990; and Anders et al., Mechanical Properties of Amorphous Hard Carbon Films Prepared by Cathodic Arc Deposition”, Mat. Res. Soc. Symp. Proc. Vol. 383, 1995. Japanese laid-open publication 62-183022 discusses using a plasma CVD process to make a carbon film on a magnetic disk. Weiler, Kang, Robertson, the '691 patent, Brown, Sanders, Anders, and the 62-183022 references are incorporated herein by reference.
SP3 carbon has an atomic structure that differs from SP2 carbon. Accordingly, the behavior of SP2 carbon can be quite different from SP3 carbon—sometimes to an unpredictably great extent.
As mentioned above, magnetic disk drive 12 contains magnetic disk 10 with carbon protective overcoat 22 and lubricant 24 applied to the disk. The disk substrate 14 is textured to minimize friction and stiction between disk 12 and read-write head 26. The disk/read-write head interface constitutes a finely tuned tribological system designed to minimize static and dynamic friction and wear. The texturing of the disk, the composition, deposition conditions and structure of carbon protective overcoats 22 and 34, the other elements added to the carbon overcoats, the types of lubricants, the additives in the lubricants, lubricant application process and related parameters are determined based on exhaustive research to ensure that the disk drive can survive a large number of on/off (contact-start-stop, or “CSS”) cycles. Changing one element in this tribological system call alter the behavior of the entire system. For example, if one were to replace a conventional type of predominantly SP2 carbon with a different type of carbon, e.g. a predominantly SP3 carbon, that can completely change the behavior of the tribological system.
Merely by way of example, it has been discovered that when one tries to use the '107 type carbon and a perflouropolyether lubricant such as Z-dol (manufactured by Montedison Co. of Italy) mixed with an X1P additive, for reasons not well understood, the resulting disks tend to fail glide tests. This is particularly interesting and unexpected, since the lubricant thickness is only about 3 nm, whereas the glide testing is performed at a glide height of about 1 microinch, or about 25 nm. Thus, it is highly unexpected that the lubricant could interact with the carbon film in such a way as to cause a failure in a glide test where the glide height is eight times the lubricant thickness.
Certain forms of high SP3 carbon formed by chemical vapor deposition have been found to exhibit other problems, i.e. sensitivity to certain types of contaminants.